Explicitly request 24bpp screen with Xvfb

Older versions of the server default to 8bpp, which breaks some of our
tests (admittedly, this shouldn't be the case, but it's probably better
to test in the same configuration as is used by 99.99% of the users
rather than adjusting or disabling tests in the remaining 0.01%).

build/tools/travis-ci.sh

@@ -68,7 +68,7 @@ case $wxTOOLSET in
 
         if [ "$wxUSE_XVFB" = 1 ]; then
             echo 'Testing GUI using Xvfb...' && echo -en 'travis_fold:start:script.testing_gui\\r'
-            pushd tests && xvfb-run -a ./test_gui && popd
+            pushd tests && xvfb-run -a -s '-screen 0 1600x1200x24' ./test_gui && popd
             echo -en 'travis_fold:end:script.testing_gui\\r'
         fi